I am new here and I would like some help to design an opamp for my D/A converter. I would like to design with cmos transistors a R-2R architecture converter. I did it with a model of opamp, but now I need to design the opamp with cmos. And I do not know how to do. Could you help me ?
I thank you...

Is this a college course that you are currently taking? What other circuit elements have you designed?

An opamp typically requires multiple circuit elements put together to get the overall desired result. You will typically need three stages. 1) a differential input stage, 2) an inverting stage, and 3) an output stage. Also, when you are designing in CMOS you will need to set up a reference current source to be used across the design.

1) How are you biasing the inputs to the op amp? From the R-network page it looks like the bias is around GND. The op amp lower supply also seems to be GND which would probably never turn on M4. INP needs to be higher than TEST0 to turn it on. Is that what you are seeing in the simulations? You may need to add a -Vcc supply or bias the input at 0.5 Vcc. If you bias at half supply you have to make sure your R network does the same thing.

2) Are you going to need an output stage? The 1 kohm feedback resistor may affect the output and frequency response of the circuit.

As StayatHomeElectronics implied, your op amp's input common mode range does not include the negative rail (ground). There are designs for rail-to-rail I/O op amps, but they are much more complex. If you want to run your inputs at ground, you will need a negative supply.

What is the value of R in your R-2R ladder?

EDIT: if you change all the N-channels to P-channels and vice-versa (only in the op amp), your input common mode range can include ground.

You are right, my inputs are biased like this:
INN = signal from the R-2R ladder
INP = gnd
My supply voltages for the whole circuit are Vss=0V VDD=5V and I am not supposed to use a negative one.
The value of R is 1kohm

I tried to bias at 2.5V the INP input as you suggested to me, but the signal was worse.
I need the feedback resistor to convert the current into voltage. Is it what you call "output stage"?

I did a changement in the R-2R ladder switch. Instead of using pMos, I used nMos with an inverter before. Because I noticed that their resistances were not equal and so I did not have a symmetrical current through the transistors. Now I have a symmetrical current through them, but the behaviour of the converter remains incorrect.
I think it could be due to the opamp which may be not adapted. But I do not know what I can change on it....